“We urge you to use all available means to avert a catastrophic military assault on Yemen’s major port city of Hodeida by the Saudi-led coalition, and to present Congress with immediate clarification regarding the full scope of U.S. military involvement in that conflict,” said a draft of the letter obtained by The Hill.

“In light of your April 2017 remarks that the war must be resolved ‘politically as soon as possible,’ we urge you to use all tools at your disposal to dissuade the Saudi-led coalition from moving forward with this offensive and reject the provision of U.S. logistical, military and diplomatic support for any such operation.”

The letter comes after the United Nations and humanitarian groups reported over the weekend that they were warned by the United Arab Emirates to evacuate the Yemeni port city of Hodeida by Tuesday. The city is controlled by Houthi rebels who are fighting a Saudi Arabia-led coalition, and the coalition believes the port has been key to the rebels smuggling in arms.

Humanitarian groups and experts have warned that an offensive on Hodeida, through which 80 percent of Yemen’s aid comes, could devastate the already war-ravaged country. The United Nations said Friday the worst-case scenario is 250,000 civilians killed in the assault.

The House lawmakers are asking their colleagues to join their “urgent call” to Mattis in light of the potential Hodeida offensive.

“U.S. involvement in the Saudi-UAE-led war against Yemen’s Houthi rebels has been shielded from public scrutiny and congressional oversight,” they wrote in the “Dear Colleague” letter asking for signatures.

The United States supports the Saudi campaign with billions of dollars in arms sales, intelligence sharing and logistics such as air refueling. The New York Times also reported last month that Army Green Berets are at Saudi Arabia’s border helping find and destroy Houthi missile launchers.

Lawmakers in both chambers of Congress have gotten increasingly frustrated at the Saudi campaign as the civilian death toll rises, most of which has been blamed on Saudi airstrikes.

Late last year, the House passed a nonbinding resolution that called U.S. military involvement in the war unauthorized.

Earlier this year, the Senate blocked a resolution that would have ended U.S. military support to the campaign, though the vote margin was narrower than expected. Following that vote, a provision was added to the Senate version of an annual defense policy bill that would make U.S. refueling of coalition aircraft conditional on the coalition meeting certain criteria.

In the letter to be sent to Mattis, the House lawmakers express concern the Pentagon misled Congress about the U.S. role in Yemen during debate on the Senate resolution in light of the Times report.

“We call on you to immediately disclose the full extent of the U.S. military role in the Saudi-led war against Yemen’s Houthis,” the draft said, “including the use of special operations forces; disclose any role that the Pentagon is currently performing, has been asked to perform, or is considering performing regarding an attack on the port of Hodeida; and issue a public declaration opposing this impending assault and restating the administration’s position that Saudi Arabia and other parties to the conflict should accept an immediate ceasefire and move toward a political settlement to resolve the conflict.”